1. Field of the Invention
The present invention relates to an LED (Light-Emitting Diode) driver, and more specifically to a programmable LED driver with an embedded non-volatile memory storing control data for custom programming of a variety of features of the LED driver.
2. Description of the Related Arts
White LEDs are being used increasingly in display devices. For example, some modern liquid crystal display (LCD) devices use white LEDs as the backlight for the LCD display. These LEDs are typically driven by an LED driver. White LED drivers are typically constant current devices where a constant sink current is fed through the white LEDs to provide a constant luminescence. The anode of the white LEDs is driven by a charge pump circuit.
FIG. 1 illustrates a conventional LED driver 100 driving LEDs 112, 114. For example, the LEDs 112, 114 can be white LEDs. The LED driver 100 includes 2 main circuit blocks, a charge pump 102 and a current regulator 110. The charge pump 102 typically converts a battery voltage (VIN) into an output voltage (VOUT) coupled to the anodes of the LEDs 112, 114. The output voltage (VOUT) drives the LEDs 112, 114.
Current through the LEDs 112, 114 sets their intensity and associated luminescence. Thus, in order to obtain accurate intensity, which is very important for displays, the current through the LEDs 112, 114 must be set accurately. Typically, the current regulator 110 is responsible for driving the LEDs with constant current. The current regulator 110 includes, among other components, a bandgap voltage generator 104, an error amplifier comprised of the amplifier 106 and the transistor 119, a current mirror 108 comprised of transistors 116, 118, and LED drive transistors 122, 124, 126.
The bandgap voltage generator 104 generates a bandgap voltage Vref, and the error amplifier (106, 119) ensures that the voltage at node 121 across the resistor REXT 120 is set at Vref. Typically, the resistor REXT 120 is external to the LED driver circuit 100. The reference current IREF through the external resistor REXT 120 is set by the bandgap voltage Vref and the external resistor REXT 120. That is, the reference current IREF is set by Vref/REXT. The reference current IREF is repeated through the transistor 122 by the current mirror 108, and eventually drives the LEDs 112, 114 by the transistors 122, 124 and the transistors 122, 126, respectively. The size (W/L ratio, or width/length ratio) of the transistors 124, 126 relative to the size of the transistor 122 determines how large the current ID1, ID2 through the LEDs 112, 114 is relative to the reference current IREF through the transistor 122. Thus, the current ID1, ID2 through the LEDs 112, 114 is also determined by the bandgap voltage Vref and the external resistor REXT 120. The resistance REXT of the external resistor 120 needs to be set accurately in order to control the luminescence of the LEDs 112, 114 precisely. In conventional LED drivers 100, there is no convenient way to change the current through the LEDs 112, 114 without changing the resistance value of the resistor 120.
Typical LED drivers 100 may use an external resistor 120 to set the current in the LEDs 112, 114. Such external resistor 120 adds a pin to the LED driver IC (integrated circuit), extra board space for the overall LED driver circuitry, and results in an increase in the Bill-of-Materials (BOM) cost for the overall LED driver circuitry. Note that different applications might require different maximum currents from the LED driver 100. This is because different LEDs 112, 114 from different manufacturers may give different intensity for different current values. With a conventional LED driver 100, the only way to control the reference current IREF is to change the resistance value of the external resistor 120 so that the current through the LEDs 112, 114 change accordingly. The resistor 120 is typically external to the LED driver 100 in order to have its resistance value changed, which results in waste of a pin, board space, and cost, as explained above.
The charge pump 102 typically operates in multiple operation modes. Initially at power up of the LED driver 100, the input voltage VIN is attached to the output voltage VOUT via the charge pump 102 so that VIN equals VOUT. This mode is often called the 1× mode. The charge pump 102 typically changes operation modes as time goes by and the battery voltage VIN drops over time, because the LEDs 112, 114 typically have a voltage drop. The typical voltage drop VLED in a white LED may be, for example, 3.4 V.
As the input voltage VIN decreases over the lifetime of the battery (not shown), the output voltage VOUT decreases in the same proportion since VIN equals VOUT when the charge pump is in 1× mode. Thus, the voltage at nodes 115, 117 (the LED driver pins) is given by VOUT−VLED. When the voltage at nodes 115, 117 becomes too low, typically 200 mV, the current regulator 110 goes out of saturation and can no longer provide an accurate current through the LEDs 112, 114. This causes the charge pump 102 to switch to a higher operation mode, typically a 1.5× mode that generates the output voltage VOUT to be 1.5×VIN. As a result, the LED driver pin voltage at nodes 115, 117 rises high enough to push the current regulator 110 back into saturation. This process is repeated, and when the battery voltage VIN further decreases to cause the current regulator 110 to go out of saturation even under 1.5× mode, the charge pump switches to 2× mode that generates the output voltage VOUT to be 2×VIN.
Although the charge pump 102 may automatically switch to different operation modes as explained above, some LED applications may need to set the operation mode of the charge pump 102 to a single operation mode or have only selected ones of multiple operation modes, even when the charge pump 102 itself has circuitry to operate in multiple operation modes. In order to set the operation mode of the charge pump 102 in a conventional LED driver 100, fixed circuitry has to be used in the charge pump 102 to permanently set the operation mode, which essentially requires manufacturing different LED driver integrated circuits using different metallization processes during the fabrication process of the LED driver IC.
Therefore, there is a need for a more convenient technique to change the maximum current through the LEDs. There is also a need for a technique to bring the resistor for generating the reference current internal to the LED driver and be able to trim the resistor. Finally, there is a need for a more convenient technique to set the operation mode of the charge pump of the LED driver.